Porous aluminum, as the representative of metal foam, is a multifunctional material with lightweight property and excellent bombarding-energy absorbing property, and is expected to serve as a high-functional material capable of ensuring compatibility between improving fuel efficiency and improving collision safety of automobiles.
Many methods for producing the porous aluminum have been proposed until now; among these propositions, a precursor method is a representative one (see Non-patent document 1). The precursor method is adapted to produce a so-called precursor (a metal foam precursor) in which blowing agent powder is previously mixed into an aluminum base material. In the precursor method, after the precursor has been formed, the precursor is heated so that the blowing agent is decomposed to generate gas, and the aluminum base material is blown out by the gas to become a porous material. By the above process, production of the porous aluminum is completed.
Further, as examples of the precursor method for producing the precursor, there are a powder metallurgical method (see Non-patent document 2, Patent document 1, and Patent document 2) and a rolling-bonding method (see Non-patent document 3 and Patent document 3).
In the powder metallurgical method, base material powder and blowing agent powder are mixed until they become homogeneously mixed powder, and further, the mixed powder is subjected to a number of operations such as hot extrusion, hot rolling and the like, so that the mixed powder is solidified to become a precursor.
In the rolling-bonding method, a plurality of plate materials (as base materials) are prepared, a blowing agent is sandwiched between the plate materials, and the plate materials are rolled at a predetermined rolling reduction of 50%, for example, so that the plate materials are joined to each other. Further, the rolling process is repeatedly performed so that the blowing agent homogeneously disperses within the plate materials, to thereby obtain a precursor.